The invention relates to plastic encapsulation of electronic devices, and more specifically, to injection molding an encapsulation for an electronic device directly onto a substrate such as a printed circuit board.
It is well known that electronic devices are sensitive to the environment and that exposure to normal atmospheric conditions may degrade or ruin them entirely. Accordingly, it is the current practice to protect electronic devices from environmental/atmospheric exposure by sealing them within a protective enclosure, commonly made of a non-electrically conducting material such as a plastic resin, with an interfacing means, such as pins, to allow connection of the devices to a larger electronic circuit or other devices. Simple devices such as resistors, capacitors, diodes and the like, as well as more complex semiconductor devices, or chips, are commonly packaged in this manner.
It is common practice to interface such an encapsulated device with other devices mounted on a supporting substrate by, for example, inserting its interface pins into a corresponding socket mounted on the substrate. These other devices are similarly mounted and connected to each other with wires, or with traces in the case where the substrate is a printed circuit board.
This practice of encapsulation suffers from a number of drawbacks. Generally, the equipment and materials necessary to accomplish the encapsulation must be located outside of the clean room environment where the device itself is manufactured, and the encapsulation must therefore be performed as a separate manufacturing step. The encapsulating process is also expensive. Further, the plastic packages themselves, with the required interface means, significantly increase the size of the device, thereby requiring a larger area, or more real estate, for their incorporation in another device or circuit.
Alternatively, it is known that certain electronic devices may be mounted to a substrate such as a printed circuit board, typically with gold wire connections, and encapsulated by a liquid resin that is hand cast over the device on the substrate. This procedure is not preferred because it is expensive, time-consuming, difficult to accurately place the cast material over the device, and provides poor adhesion of the cast material to the substrate. Further, the gold wire connections to the device are very delicate and are easily disconnected during the casting process.
Another prior-art method for encapsulating devices mounted onto a substrate by gold wires is the so-called "transfer molding" method. Transfer molding is a process by which a thermosetting material is caused to flow into a cavity formed by the cooperation of a mold and a cavity plate. The material enters the cavity through so-called "side" gates which are also formed by the space between the cavity plate and the mold. This method is an improvement over the hand-casting encapsulation method because it allows multiple devices to be encapsulated at the same time in one production cycle and it allows for somewhat more accurate placement and size of the resulting encapsulated package.
Transfer molding, however, itself suffers from a number of significant drawbacks which are eliminated by the present invention. Initially, transfer molding techniques of encapsulating electronic devices are limited to use of thermosetting materials which have a low viscosity. Such a material is necessary to prevent damage to the delicate connections of the device to the substrate during the molding process. This same danger requires that the encapsulating material be forced into the mold at low pressure. Use of a low viscosity thermoset results in the need for an expensive mold apparatus which must be constructed with very high tolerances to prevent leakage of the encapsulating material. Even the most expensive molds, however, exhibit some leakage in the area of the gate and device connections which must be removed by additional process steps after molding, thereby increasing cycle times.
Use of a thermoset, which cures by a chemical process, also results in long cycle times, on the order of 5 to 15 minutes, which increases production costs. Thermoset materials themselves are expensive due, in part, to the inability to reuse excess encapsulating material resulting from the molding process after the material has cured.
Use of a low viscosity thermoset at low insertion pressure also results in the need for large side entry gates for the encapsulating material. The large side gates make transfer molding impractical for small devices because the size of the gate limits the size of the cavity. The gating used in transfer molding techniques adds further limitations to the placement and configuration of the devices to be encapsulated because it requires the devices to be near an edge of the substrate to which it is to be bonded. Typical transfer molding applications therefore involve devices which are mounted in a linear arrangement on a substrate with the use of strip-like carriers, or "lead frames," for the devices. Transfer molding with thermosets is also not useful with small devices because the thermosetting material requires substantial surface area in contact with the substrate in order to adhere sufficiently to hold the device and encapsulate to the substrate with a chemical or adhesive bond.
On the other hand, use of higher viscosity thermoplastic materials is not practical in transfer molding because it requires higher pressures that may damage the device connections and may result in additional leakage of the encapsulating material. Further it is difficult in a transfer molding apparatus to maintain the high temperatures required to allow a thermoplastic material to properly flow.
Attempts have been made to solve the problems with prior-art encapsulating methods by use of injection molding. Prior art injection molding methods, however, suffered from similar drawbacks. Although higher pressures may be used with injection molding and thus would allow use of thermoplastic materials, the injection process would damage the delicate device connections. In addition, prior art injection molding methods and devices were not useful for small devices because the smaller gating necessitated by smaller cavities had a tendency to clog with the thermoplastic material and this material exhibited poor adhesion resulting in devices being separated from the substrate.